1. Field of the Invention
The present invention relates to a color management method for the conversion of internal color appearance space to color values in CMYK device space.
2. Description of the Related Art
Typically, color management systems utilize color transforms to convert colors from device-dependent to device-independent color spaces, and vice versa. This is beneficial because different devices display colors in different ways. For example, when one type of RGB monitor displays the color R=100, G=0, B=0, the measured output of that monitor may be different from the measured output of another type of RGB monitor that is displaying the same RGB value. Therefore, the two monitors cannot use the same RGB device values if display of the same color is desired. Instead, conversions are applied between device-dependent RGB values and a device-independent color appearance space so that colors can be accurately mapped and reproduced between different devices.
This mapping is typically achieved with color transforms. Conventionally, color transforms are created by measuring the output of a specific device at a plurality of known inputs. For example, a color transform for a CMYK printer can be computed by measuring color patches printed by the printer at a plurality of CMYK values. The measurements are taken by a color measuring device, such as a calorimeter or a spectrophotometer, and the measurements are in a device-independent color space, such as CIEXYZ, CIELab, CIECAM, or CIELuv. With these measurements, a color management system is able to easily predict what device-independent color a printer will produce at a specific value of CMYK.
However, color management systems ordinarily rely on a conversion in the reverse direction, from device-independent color values to CMYK device-dependent values, and this conversion is not as straightforward. As with other color spaces having four or more color components, the CMYK space is overspecified. In other words, the same color can theoretically be produced using different amounts of black ink (K) with varying amounts of cyan, magenta, and yellow (CMY). Therefore, a one to one conversion from color appearance space to CMYK space is not mathematically possible. This leads to potentially unsatisfactory conversions, since certain levels of black ink may produce visually unsatisfactory colors. For instance, neutral colors, such as grays, typically look better with higher amounts of black ink, even though mathematically accurate conversions can be found with low amounts of black ink. On the other hand, saturated non-neutral colors appear sharper with lower amounts of black ink.
Attempts have been made to control the K level in CMYK to CMYK device conversions. However, these attempts have failed to address the problems associated with controlling the K level in a color appearance space to CMYK space conversion. Other attempts have been made to treat the conversion of neutral and non-neutral color appearance space values differently, but have generally been unsatisfactory.
U.S. patent application Ser. No. 10/841,184, filed on May 6, 2004, presents a method for controlling the K value in internal color space to CMYK conversions. While the method described in that application performs well for computer-generated graphic images, improved control over the K channel is needed for natural images that include more continuous colors.